DB-25 RS232 Pinouts

To interface to the ASE Bell 202 Modem, in either the Stand-alone or Rack mount configuration, a DB-25 cable is required. The modems all have a DB-25 Female adaptor, which means the cable required to interface to the modem must be a DB-25 Male. The Pinout for the RS232 interface into the modem is as follows:

DB-25	Signal Description
2	TXD – Transmit Data
3	RXD – Receive Data
4	RTS – Request To Send
5	CTS – Clear To Send
6	DSR – Data Set Ready
7	Com – Common
8	DCD – Data Carrier Detect
20	DTR – Data Terminal Ready
22	RI – Ring Indicator (Not Required)

If the modem must be connected to a DB-9 RS232 device, the cable should 'Ring Out' as follows (pin for pin connections):

DB-9	DB-25	Signal Description
1	8	DCD – Data Carrier Detect
2	3	RXD – Receive Data
3	2	TXD – Transmit Data
4	20	DTR – Data Terminal Ready
5	7	Com – Common
6	6	DSR – Data Set Ready
7	4	RTS – Request To Send
8	5	CTS – Clear To Send
9	22	RI – Ring Indicator (Not Required)

This type adaptor should be used whenever it is necessary to convert between a DB-25 and DB-9 connector.

DB-25 Null Modem Adaptor (Standard)

The following table illustrates the pin-out of a “standard” DB-25 to DB-25 Null Modem Adaptor.

Signal Description	DB-25M	DB-25F	Signal Description
Protective Ground	1	1	Protective Ground
TXD – Transmit Data	2	3	RXD – Receive Data
RXD – Receive Data	3	2	TXD – Transmit Data
RTS – Request To Send**	4	8	DCD – Carrier Detect
CTS – Clear To Send**	5	8	DCD – Carrier Detect
DSR – Data Set Ready	6	20	DTR – Data Terminal Ready
COM – Common (Sig. Gnd)	7	7	COM – Common (Sig. Gnd)
DCD – Carrier Detect	8	4	RTS – Request To Send**
DCD – Carrier Detect	8	5	CTS – Clear To Send**
DTR – Data Terminal Ready	20	6	DSR – Data Set Ready

**NOTE: Pins 4 and 5 (RTS, CTS) on both the DB-25 Male and DB-25 Female side are tied together and connected to pin 8 (DCD) of the other side.

DB-9 Null Modem Adaptor (Standard)

The following table illustrates the pin-out of a “standard” DB9 to DB-9 Null Modem Adaptor.

Signal Description	DB-9M	DB-9F	Signal Description
DCD – Data Carrier Detect	1	7	RTS – Request To Send**
DCD – Data Carrier Detect	1	8	CTS – Clear To Send**
RXD – Receive Data	2	3	TXD – Transmit Data
TXD – Transmit Data	3	2	RXD – Receive Data
DTR – Data Terminal Ready	4	6	DSR – Data Set Ready
COM – Common (Sig. Gnd)	5	5	COM – Common (Sig. Gnd)
DSR – Data Set Ready	6	4	DTR – Data Terminal Ready
RTS – Request To Send**	7	1	DCD – Data Carrier Detect
CTS – Clear To Send**	8	1	DCD – Data Carrier Detect

**NOTE: Pins 7 and 8 (RTS, CTS) on both the DB-9 Male and DB-9 Female side are tied together and connected to pin 1 (DCD) of the other side.

You may contact us in Campbell, CA by phone at 408-364-0500, fax to 408-364-0550, or via email at products@ase-systems.com.

Normal business hours are 9:00AM to 6:00PM Pacific Time, M - F.

Applied Systems Engineering, Inc.
1671 Dell Avenue, Suite 200
Campbell, California 95008
Phone: (408) 364-0500 • Fax: (408) 364-0550
Email: products@ase-systems.com

If you would like additional information on any of the ASE products or would like to discuss your specific requirements with a sales engineer, you can call directly or fax/e-mail the information about your requirement and a sales engineer will contact you.